Age-related neurodegenerative diseases such as Alzheimer's disease (AD) and dementia are one of the largest societal challenges today. The World Health Organization estimates that costs for care of the elderly will continue to increase and that the number of diagnosed dementia cases will triple by 2050 (World Health Organization and Alzheimer's Disease International—Status Report (2012) DEMENTIA: A public health priority, WHO). The first treatments for AD were neurotransmitter modulators such as acetylcholine esterase inhibitors and NMDA modulators. These therapies became available at the turn of the millennium and still form the cornerstone for symptomatic relief of memory deficits related to dementia and AD. However, these drugs do not target the underlying causes of AD: accumulation of amyloid-43 (AP) peptide and tau protein aggregates and associated loss of neuronal synapses and eventually neurons.
Longitudinal, community-wide studies of the elderly (Weiner, M. W. et al. (2014) ADNI online: http://www.adni-info.org/; Breteler, M. M. et al. (1992) Neuroepidemiology 11 Suppl 1, 23-28; Launer, L. J. (1992) Neuroepidemiology 11 Suppl 1, 2-13) together with large genome-wide association studies (Lambert, J. C. et al. (2013) Nat. Genet. 45, 1452-1458) have shown that AD is a heterogeneous mix of dementias where up to 10 percent of the advanced AD patients lack amyloid pathology (Crary, J. F. et al. (2014) Acta Neuropathol. 128, 755-766). Furthermore, seminal pathological studies by Braak & Braak (Braak, H. and Braak, E. (1996) Acta Neurol. Scand. Suppl 165, 3-12) demonstrated a clear correlation between the degree of neurofibrillary tangle pathology and cognitive state prior to autopsy. These observations have been reinforced by several investigators (Nelson, P. T. et al. (2012) J. Neuropathol. Exp. Neurol. 71, 362-381), and in recent longitudinal biomarker studies, which indicate that cerebrospinal fluid (CSF) levels of tau and hyperphosphorylated tau increase throughout early and late stages of the disease (Jack, C. R., Jr. et al. (2013) Lancet Neurol. 12, 207-216).
As indicated above, the microtubule-associated protein, tau, and its hyper-phosphorylated version, form the main constituent of intracellular neurofibrillary tangles, which are one of the main hallmarks of AD. Furthermore, specific genetic variants of tau are associated with familial forms of fronto-temporal dementia (FTD). Appearance of tau pathology in AD occurs in a distinct spatial pattern, starting in the entorhinal cortex, followed by hippocampal and cortical areas (Braak, H. and Braak, E. (1996) Acta Neurol. Scand. Suppl 165, 3-12). The specific stage of tau pathology also correlates well with cognitive abilities (Nelson, P. T. et al. (2012) J. Neuropathol. Exp. Neurol. 71, 362-381; Braak, E. et al. (1999) Eur. Arch. Psychiatry Clin. Neurosci. 249 Suppl 3, 14-22). Taken together, this evidence forms the basis of a tau-based hypothesis for AD. It entails that the intracellular accumulation of tau leads to microtubule degeneration and spinal collapse. As a result, communication between neurons malfunctions and cell death follows. Recently, it has also been shown that tau itself may form an endo-pathogenic species that can transmit neurodegeneration from one cell to the next (Clavaguera, F. et al. (2009) Nat. Cell Biol. 11, 909-913).
I. Tau as an Endo-Pathogen
Clavaguera and colleagues have demonstrated that tau itself may act as an endo-pathogen (Clavaguera, F. et al. (2009) Nat. Cell Biol. 11, 909-913). Low spin brain extracts were isolated from P301S tau transgenic mice (Allen, B. et al. (2002) J. Neurosci. 22, 9340-9351), diluted and injected into the hippocampus and cortical areas of young ALZ17 mice. The ALZ17 mouse is a tau transgenic mouse line which only develops late pathology (Probst, A. et al. (2000) Acta Neuropathol. 99, 469-481). The injected ALZ17 mice quickly developed solid filamentous pathology, and administration of tau immuno-depleted brain extracts from P301S mice or extracts from wild type mice did not induce tau pathology. Fractionation of the brain extracts in soluble (S1) and sarcosyl-insoluble tau (P3) (Sahara, N. et al. (2013) J. Alzheimer's. Dis. 33, 249-263) and injection of these into ALZ17 mice demonstrated that the P3 fraction is most competent in inducing pathology. It contains most of the intracellular hyper-phosphorylated filamentous tau. The majority of pathology could also be induced when injecting P301S extracts into the brains of wild type mice, but no NFTs were formed. In subsequent studies, Clavaguera et al. have shown that human tau extracted from post-mortem brain tissue of other tauopathies (Argyrophilic Grain Disease (AGD), Progressive Supranuclear Palsy (PSP), and Corticobasal Degeneration (CBD)) may also induce tau pathology in the ALZ17 model (Clavaguera, F. et al. (2013) Proc. Natl. Acad. Sci. U.S.A. 110, 9535-9540). Since the presentation of these data, several other tau seeding and spreading models have been reported (Ahmed, Z. et al. (2014) Acta Neuropathol. 127, 667-683; Walker, L. C. et al. (2013) JAMA Neurol. 70, 304-310). The main conclusion from these studies indicates a mechanism by which pathogenic tau in intracellular inclusions is secreted from the cell into the periplasmic space. The pathological tau material is then transported along the vesicular sheath in both anterograde and retrograde direction and subsequently taken up by neighboring cells by means of bulk endocytosis. This mechanism explains why the spread of pathology observed in human disease follows a distinct anatomical pattern. Intriguingly, peripheral administration of pathological tau may accelerate the formation of tau pathology in ALZ17 mice (Clavaguera, F. et al. (2014) Acta Neuropathol. 127, 299-301). This spreading mechanism may explain disease propagation in other proteinopathies (Goedert, M. et al. (2010) Trends Neurosci. 33, 317-325; Sigurdsson, E. M. et al. (2002) Trends Mol. Med. 8, 411-413).
II. Tau Species
The discovery that the tau protein may act as an endo-pathogen has spawned a search for “The Pathogenic Species” that could be targeted in potential interventive therapies.
The microtubule-associated protein tau gene (MAPT) is located on chromosome 17 of the human genome and expresses six isoforms of the tau protein in adult human brain. These isoforms arise from the alternative splicing of exons 2, 3 and 10 of the 16 exons within the MAPT gene. Exons 2 and 3 express a 29-amino acid repeat and exon 10 expresses an additional microtubule binding domain. As a result, tau isoforms will contain 0, 1 or 2 N-terminal repeats and 3 or 4 C-terminal microtubule binding domains (3R or 4R tau). Commonly six isoforms of tau are expressed. The longest (2N4R) and shortest (ON3R) isoforms consist of 441 and 352 amino acids, respectively (Kolarova, M. et al. (2012) Int. J. Alzheimers. Dis. 2012, 731526). The N-terminal projection domain of tau (2N4R) consists of a 44-amino acid glycine-rich tail and residues 45-102 encompass two highly acidic regions (N1, N2-domains). Two proline-rich regions are found at residues 151-243 (P1, P2 domains). The remainder of the protein is constituted by four microtubule binding domains (R1-R4), followed by a short C-terminal region.
Tau is soluble and highly phosphorylation-labile protein. Approximately 20 percent or 85 amino acid residues in the longest isoform of tau are potential (Ser, Thr or Tyr) phosphorylation sites. Approximately half of these have been observed to be phosphorylated experimentally (Hanger, D. P. et al. (2009) Trends Mol. Med. 15, 112-119; Hasegawa, M. et al. (1992) J. Biol. Chem. 267, 17047-17054), and the phosphorylation sites are clustered around the terminal residues of the microtubule binding domains. Tau is dynamically phosphorylated and de-phosphorylated during the cell cycle. It must dissociate from microtubules to allow for mitosis to occur. Its main role in post mitotic cells (the differentiated neuron) is to act as a microtubule stabilizer, allowing for optimal axonal transport. It can only associate with microtubules in its mostly de-phosphorylated form, thus phosphorylation acts as a direct microtubule association/dissociation switch within the neuron. Under normal conditions, cytosolic tau contains on average two phosphorylated sites. In paired helical filamentous material, at least 7-8 sites are phosphorylated (Hanger, D. P. et al. (2009) Trends Mol. Med. 15, 112-119; Hasegawa, M. et al. (1992) J. Biol. Chem. 267, 17047-17054). Hyperphosphorylated, paired helical filamentous tau is a key hallmark of Alzheimer's disease (Kosik et. al. (1986) PNAS, 86, 4044-4048), a distinct mobility shift of hyperphosphorylated tau is observed in immune-cytochemical analysis of human AD brain material.
It has been difficult to study the tau protein with traditional structural techniques like x-ray crystallography or NMR spectroscopy, reflecting its meta-stable nature. Such studies have mainly been conducted on domain fragments of the un-phosphorylated tau protein. The only structural study to date on full-length tau (2N4R), using NMR spectroscopy, reveals that the protein contains only sparse stretches of stable secondary structure (Mukrasch, M. D. et al. (2009) PLoS. Biol. 7, e34). This analysis indicates that the secondary structure of the peptide backbone has a large propensity for adapting a 3-sheet structure. The backbone's first 200 residues are considerably more ordered than the C-terminus encompassing the microtubule binding domains. The presence of many specific long-range interactions within the protein in solution indicates that it exists in a largely disordered molten globular state (Ohgushi, M. and Wada, A. (1983) FEBS Lett. 164, 21-24).
Protease products of tau generated in particular by caspase and calpain (Asp13, Glu391 and Asp421) have been identified in tangle material (Gamblin, T. C. et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 10032-10037). In particular, the truncation at Asp421 has been studied in detail using the tau C3 antibody, which binds to the free Asp421 terminus. This truncation has been postulated as an early event in AD pathogenesis associated with induction of apoptosis (deCalignon A. et al. (2010) Nature 464, 1201-1204). The N-terminal cleavage at Asp13 and the C-terminal cleavage at Glu391 are considered late events in the pathogenesis (deCalignon A. et al. (2010) Nature 464, 1201-1204; Delobel, P. et al. (2008) Am. J. Pathol. 172, 123-131). Recently, an additional N-terminal fragment (residues 1-224) was identified in CSF from AD and PSP patients, and has been hypothesized to be an early marker of disease and particularly pathogenic (U.S. Ser. No. 14/092,539; Bright, J. et al. (2014) Neurobiol. Ageing, 1-17). A similar calpain cleaved fragment was reported by other groups (Ferreira, A. and Bigio, E. H. (2011) Mol. Med. 17, 676-685; Reinecke, J. B. et al. (2011) PLoS. One. 6, e23865).
Apart from hyper-phosphorylation and tau fragmentation, post-translational acetylation (Cohen, T. J. et al. (2011) Nat. Commun. 2, 252; Min, S. W. et al. (2010) Neuron 67, 953-966) and O-GlcNAcylation (Zhu, Y. et al. (2014) J. Biol. Chem.) have been proposed to be pathology defining processes in the formation of tangle pathology associated with AD.
III. Tau Immunotherapies
Immunotherapies are traditionally separated into passive and active vaccine approaches. In an active vaccine approach, a pathogenic agent or an inactivated pathogenic form thereof, is injected into the patient and the immune system elicits an immune response. This triggers the maturation of B-cells generating high affinity antibodies or cellular response against the administered antigen. In a passive vaccine approach, the triggering of the immune system is circumvented by infusing a specific antibody against the antigen. The inherent clearance system then removes antibody-bound ligand.
AC Immune is pursuing a mouse monoclonal antibody against phospho-serine 409 of tau. Antibodies were profiled against human AD and control brain tissue and were selected based on their ability to recognize tangle pathology. The humanized version of two antibodies, hACI-36-2B6-Ab1 and hACI-36-3A8-Ab1, both bind to a tau epitope within amino acids 401-418 (WO 2013/151762).
The group of Roger Nitsch has isolated tau auto-antibodies from elderly healthy individuals with no sign of degenerative tauopathy. A number of antibodies have been isolated using full length recombinant human tau (2N4R) to find tau specific antibodies. These were then screened for their ability to discriminate tau isolates from diseases and healthy individuals. Three lead antibodies, 4E4, 4A3 and 24B2, have been described in the patent literature (WO2012049570; US2012087861). Their epitope mapping indicates that all recognize amino acids within and C-terminal to the microtubule binding region, from position V339 to K369. These antibodies do not exhibit any phospho-specificity.
C2N Diagnostics focuses mainly on developing diagnostic tools for early detection of neurodegenerative disease. Antibodies were generated against full length human and mouse tau protein. Eight and five antibodies were identified, recognizing human and mouse tau, respectively (Yanamandra, K. et al. (2013) Neuron 80, 402-414). Three antibodies with different binding kinetics were selected for in vivo evaluation. Namely, HJ9.3, HJ9.4 and HJ8.5, recognizing tau residues 306-320, 7-13 and 25-30, respectively, with the last one (HJ8.5) being specific for human tau. The antibodies were also selected based on their ability to prevent transfer of pathology in an ingenious mechanistic reporter assay of trans-cellular propagation of tau (Sanders, D. W. et al. (2014) Neuron 82, 1271-1288; Kfoury, N. et al. (2012) J. Biol. Chem. 287, 19440-19451). Their evaluation in chronic i.c.v. injection studies in P301S transgenic mice demonstrated their ability to reduce levels of hyper-phosphorylated tau protein as determined in immuno-histochemical analysis of the treated mice.
The antibodies of Peter Davies were developed originally as diagnostic tools that could differentiate between pathological and normal tau in AD and control brain material (Greenberg, S. G. and Davies, P. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 5827-5831). Evaluation of the therapeutic utility of the PHF1 and MC1 antibodies was demonstrated in P301S and JPNL3 (P301L) (Boutajangout, A. et al. (2011) J. Neurochem. 118, 658-667; Chai, X. et al. (2011) J. Biol. Chem. 286, 34457-34467; D'Abramo, C. et al. (2013) PLoS. One. 8, e62402 mice). PHF1 recognizes a linear phospho-tau epitope (pS396, pS404) whereas MC1 is a conformation-dependent antibody that recognizes a structural tau epitope requiring two distinct parts of the linear sequence, an epitope within residues 46-202 and a C-terminal epitope between residues 312-342 (Jicha, G. A. et al. (1997) J. Neurosci. Res. 48, 128-132). Injection of these two antibodies in chronic 12-13-week immunization studies resulted in substantial reduction of spinal cord and brainstem pathology among other brain regions, which translated to an attenuation of the motor deficit observed in these mice. (D'Abramo, C. et al. (2013) PLoS. One. 8, e62402).
iPerian/Bristol Meyers Squibb has developed tau antibodies against a postulated pathological tau species, composed of an N-terminal fragment of tau (etau: residues 1-224), which promoted hyperactivity in induced pluripotent stem cell based neuronal cultures. A portfolio of antibodies has been developed, but characterization has focused on antibodies IPN001 and IPN002 that recognize an N-terminal epitope within residues 9-18. Accordingly, these antibodies detect elevated tau levels in CSF from staged AD and PSP patients that may be an early sign of disease. In vivo injections of the antibodies in JPNL3 (P301L) mice led to partial reversal of progressive motor deficits (U.S. Ser. No. 14/092,539).
Einar Sigurdsson reported the first program to demonstrate the efficacy of tau-based immunotherapy. An active vaccine consisting of tau peptide 379-408[pS396, pS404] together with Adju-Phos adjuvant was used to immunize JPNL3 (P301L) mice. In this study a prominent reduction of tau pathology was observed in the vaccine treated mice when compared to control animals. An attenuation of tauopathy-related motor phenotype was detected as well. Its efficacy was confirmed in a different mouse model (htau/PS1) not driven by mutant tau (Boutajangout, A. et al. (2011) AAIC 2011 (7, issue 4, Supplement edn) p. s480-s431; Congdon, E. E. et al. (2013) J. Biol. Chem. 288, 35452-35465; Gu, J. et al. (2013) J. Biol. Chem. 288, 33081-33095).
Prothena has evaluated three tau antibodies in the K3691 (K3) transgenic tau mouse and in a P301L mouse model. Antibodies with varying properties were selected for in-vivo evaluation. Two pS404-specific antibodies with different isotype (IgG1/k and IgG2a/k) or a total (pan) anti-tau antibody (IgG1/k) were injected in a chronic paradigm. K3691 mice were treated with weekly injections for 21 weeks starting at 3 weeks of age, and P301L mice were treated for 7 months with weekly injections starting at 4 months of age. A reduction in tau-positive neurofibrillary inclusions was observed in the K3 mice with the IgG2a/k pS404 antibody. Both of the pS404-specific antibodies were able to reduce the level of pS422-positive tau, whereas no reduction was observed in the pan tau antibody treated mice. These studies suggest that: 1) tau clearance may be antibody isotype-dependent, and; 2) It may be important to target a tau species that is relevant to disease, as the total-anti-tau antibody was unable to reduce hyper-phosphorylated tau (PCT/US2014/025044).
The inventors of the present invention have surprisingly found antibodies specific for the phosphorylated tau serine residue 396 (pS396) to be effective in disease models; this is in contrast to the prior art antibodies which recognize primarily the tau proteins phosphorylated at both 396 and 404 residues, phosphorylated at the 404 residue only or at other residues on tau.
The inventors have developed antibodies which furthermore have a remarkable specificity and selectivity to human pathological tau. The antibodies of the present invention show a much higher degree of specificity and selectivity towards human pathological tau over non-pathological tau compared to the antibodies of WO2013/050567 (see FIG. 1 of WO2013/050567). The antibodies of WO2012/045882 reported to have a specific binding, were elicited from 6 to 9 residue amino acid sequences of Tau amino acids 393-401, 396-401, 394-400 and 393-400. This contrasts from the antibodies of the present invention which were elicited against pathogenic hyperphosphorylated tau comprising a longer amino acid sequence as described herein.
Furthermore, the antibodies and epitope-binding fragments thereof, of the present invention show many advantageous features such as the ability to discriminate between pathological and non-pathological human tau protein, and in particular to bind tau associated with Alzheimer's (AD) pathology. In electrophysiological studies, the antibodies, and epitope-binding fragments thereof, of the invention were additionally able to reverse reduced paired pulse facilitation and spontaneous miniature excitatory synaptic current (mEPSC).